fixup: add Unicode name for underscore

Author: jensmaurer

source/basic.tex

@@ -989,7 +989,7 @@
 
 \pnum
 A declaration is \defn{name-independent}
-if its name is \tcode{_} and it declares
+if its name is \tcode{_} (\unicode{005f}{low line}) and it declares
 a variable with automatic storage duration,
 a structured binding not inhabiting a namespace scope,
 the variable introduced by an \tcode{init-capture}, or